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Mechanism of CO Disproportionation on Reduced Ceria Yi Liu, Cun Wen, Yun Guo,* Xiaohui Liu, Jiawen Ren, Guanzhong Lu, and Yanqin Wang*[A]

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Mechanism of CO Disproportionation on Reduced Ceria Yi Liu, Cun Wen, Yun Guo,* Xiaohui Liu, Jiawen Ren, Guanzhong Lu, and Yanqin Wang*[A] DOI: 10.1002/cctc.200900246 Mechanism of CO Disproportionation on Reduced Ceria Yi Liu, Cun Wen, Yun Guo,* Xiaohui Liu, Jiawen Ren, Guanzhong Lu, and Yanqin Wang*[a] CO disproportionation produces carbon deposits that cover form spectra indicate that the CÀO bond of the CO molecule active sites and induce catalyst deactivation. However, under- on Ce3+ ion is weakened. With CÀO bond dissociation, standing of this detrimental reaction on reduced ceria is asymmetrical inorganic carboxylate species are formed. These deficient. Herein, the reversibility and reaction mechanism of species are the key reaction intermediates in CO disproportio- CO disproportionation on reduced ceria are investigated. The nation and are further converted to produce CO2. EPR reversibility of the CO disproportionation was studied by CO experiments indicate that the unpaired electrons produced by pulse, isotopic oxygen tracer, thermal analysis, and CO2 pulse the reduction weaken the CÀO bond through back-donation experiments. In situ diffuse-reflectance infrared Fourier trans- of electrons. Introduction Noble metal supported catalysts have been widely studied in Herein, our study into the reversibility and mechanism of CO many important reactions, such as the reverse water–gas shift disproportionation is discussed. The reversible reaction of CO [1,2] reaction and CO2 reforming of methane. In these catalysts, disproportionation on reduced ceria was studied by the CO ceria is a widely employed support because of its unique pulse, isotopic oxygen tracer, thermal analysis (TG/DTA), and oxygen-storage capacity. The addition of ceria improves the CO2 pulse experiments. Moreover, in situ DRIFTS was conduct- performance of catalysts but these catalysts still suffer from ed after CO/CO2 treatment to study the reaction intermediates, severe deactivation. Catalyst deactivation can be attributed to and electron paramagnetic resonance (EPR) spectroscopy was several factors, such as over-reduction of ceria, formation of used to investigate the influence of the reduction on the prop- stable carbonate species, and formation of carbon deposits.[3–7] erties of the ceria. The reduction generated unpaired electrons, The carbon deposits usually arise from CO disproportiona- which weakened the CÀO bond in CO on absorbed Ce3+. tion,[1,8] which can be expressed as follows: CO+COÐC+CO2 Based on this expression, the occurrence of CO disproportiona- Results and Discussion tion involves the dissociation of CÀO bonds. CO dissociation CO pulse experiments on noble metal-loaded rare earth oxide catalysts has been ex- tensively studied.[9–12] Mullins and Overbury[10] investigated the CO pulse experiments were conducted on reduced ceria adsorption and reaction of CO on Rh-loaded ceria by soft X-ray (R-CeO2; see Experimental Section) to study the forward photoelectron spectroscopy, and found that atomic carbon reaction of CO disproportionation. If disproportionation takes [11] place, CO can be detected in the pulse experiment. The was produced by CO dissociation on Rh/CeOx. Putna et al. 2 also detected CO dissociation on highly reduced Rh-loaded results of the CO pulse are shown in Figure 1. The intensity of ceria/YSZ (YSZ =yttria-stabilized cubic zirconia), but not on Pd- the CO signal increases gradually at first, and finally reaches [9] or Pt-loaded ceria/YSZ. Holmgren et al. suggested that CO equilibrium, whereas the intensity of the CO2 signal decreases disproportionation was the most reasonable explanation for with the increasing pulse number. The consumption of CO in the first several pulses is concomitant with the production of the CO2 formation on strongly reduced Pt/CeOx. Generally, the presence of a noble metal was considered a prerequisite for CO2. The CO consumption should not be attributed solely to CO disproportionation. adsorption because adsorption cannot generate CO2. Another To our knowledge, there have been two reports of CO potential reason for CO consumption was the reaction of the disproportionation on reduced ceria. Li et al.[13] detected car- adsorbed CO with hydroxy groups to form formate intermedi- bonate signals by in situ diffuse-reflectance infrared Fourier- [a] Y. Liu, C. Wen, Prof. Y. Guo, X. Liu, Dr. J. Ren, Prof. G. Lu, Prof. Y. Wang transform spectroscopy (DRIFTS) on H2-reduced ceria, and Lab for Advanced Materials, Research Institute of Industrial Catalysis proposed that these carbonate species came from CO dispro- East China University of Science and Technology, Shanghai 200237 (P.R. portionation on the reduced ceria. Recently, Swanson et al.[14] China) detected carbon species, which were the products of CO Fax: (+ 86)21-64253824 E-mail: [email protected] disproportionation, on reduced ceria by in situ Raman spec- [email protected] troscopy. However, the reversibility and mechanism of this Supporting information for this article is available on the WWW under reaction have not, to date, been clarified. http://dx.doi.org/10.1002/cctc.200900246. ChemCatChem 0000, 00, 1 – 7 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1& These are not the final page numbers! ÞÞ Y. Guo, Y. Wang et al. These experiments were conducted by introducing isotopic oxygen into the recirculation volume. In our study, the catalyst, CeO2, was more reducible than MgO. Moreover, the experi- mental conditions in our study are more favorable for oxygen exchange than those in the aforementioned studies.[1516] The CO pulse experiment was then performed on reduced ex- changed ceria (RE-CeO2 ; see Experimental Section) to simulate the CO pulse experiment on R-CeO2 and to identify the origin of oxygen in the produced CO2. The results (Figure 2) show 16 16 18 that C O2 was the main product, and that little C O Owas 16 formed. The intensity of the C O2 signal was stronger than that of the C16O18O signal, which indicates that the 16O in the 16 C O2 was not from the sample lattice. If the CO had primarily reacted with the lattice oxygen in RE-CeO2, the intensity of the Figure 1. MS signals of outlet gas during CO pulse experiment on R-CeO2 at 16 18 4008C. C O O signal should have been stronger than that of the 16 C O2 signal, as the aforementioned results showed that the amount of 18O in the sample lattice was more than that of 16O. ates, which further convert to CO2. However, this possibility Therefore, these results indicate that the production of carbon was ruled out because the formate intermediates are not de- dioxide results from CO disproportionation on reduced ceria. tected in the in situ DRIFTS experiment. Therefore, the evolved CO2 may be from CO disproportionation or the reaction be- tween pulsed CO and lattice oxygen. To identify which reaction Thermogravimetric and differential thermal analyses was responsible for the CO2 formation, isotopic oxygen tracer To further verify the occurrence of the CO disproportionation, experiments were conducted. thermogravimetric and differential thermal analyses (TG/DTA) were carried out to establish the changes in weight and heat brought about by carbon deposit combustion. TG/DTA experi- Isotopic oxygen tracer experiments ments were performed on carbon-deposited ceria (D-CeO2 ; see Experimental Section) under oxidative conditions. If a carbon To analyze the 18O distribution in isotopic oxygen tracer deposit was present on the sample, combustion of the carbon experiment, the extent of oxygen exchange on the isotopic deposit would induce weight loss and exothermic phenomena. oxygen-exchanged ceria (E-CeO2 ; see Experimental Section) The TG/DTA curves on D-CeO2 under air are shown in was determined by the CO pulse experiment (see the Support- Figure 3. The TG plot shows two weight-loss steps. The weight ing Information, Figure A.2). The calculated ratio of the loss and endothermic peak before 2008C are attributed to amount of C16O18OtoC16O was about 3.3:1. The exchange 2 desorption of physically adsorbed water. Another steep extent of oxide catalysts has been studied in depth. Karasuda weight-loss step (ca. 0.08 %) occurred at about 6108C, and an and Aika conducted isotopic oxygen exchange (IOE) experi- exothermic peak was detected at the corresponding tempera- ments on irreducible MgO.[15] At 973 K, the rate of oxygen ex- ture in the DTA plot. The peaks at about 6108C can not be change was fast, and it reached equilibrium within 100 min. attributed to desorption of hydroxy or CO groups, because Royer et al.[16] indicated that IOE proceeded quickly on LaFeO . 3 these desorption processes are endothermic. Furthermore, The exchange reached equilibrium within 80 min at 812 K. these peaks can not be attributed to desorption of carbonate Figure 2. MS signals of products during CO pulse experiment on RE-CeO2 at Figure 3. Thermogravimetric (TG) and differential thermal analysis (DTA) 4008C. plots on D-CeO2 under air flow. &2& www.chemcatchem.org 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 0000, 00, 1 – 7 ÝÝ These are not the final page numbers! CO Disproportionation on Reduced Ceria species either, according to the results of TG/DTA under N2 is roughly equal to 1:2. This ratio agrees well with the stoichio- atmospheres (see the Supporting Information, Figure A.3). No metric ratio of CO2 to CO in the CO disproportionation reaction obvious weight loss can be attributed to desorption of the car- formula, and this result further indicates that CO production is bonate species under N2 flow in the corresponding tempera- the result of the reverse CO disproportionation reaction on [17] ture range. However, previous studies have indicated that D-CeO2. oxidation of the carbon deposit takes place in this temperature range. Therefore, the weight loss and the exothermic peaks at In situ DRIFTS about 6108C can be attributed to the carbon deposit combustion. In situ DRIFTS experiments were conducted under the reactant The production of CO2 and carbon deposit was detected by atmosphere to study the detailed reaction mechanism of CO CO pulse, isotopic oxygen tracer, and TG/DTA experiments, disproportionation (Figure 5 and Table 1).
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